The invention relates generally to chemical indicators used to monitor the adequacy of a liquid sterilization process, and more specifically to chemical indicators that exhibit a uniform, easy to read color change when exposed to sterilizing conditions. An objective test for measuring color uniformity in chemical sterilization indicators is also described.
A reliable supply of sterile instruments and supplies is vitally important to modern medical practice. Various types of apparatus are known for sterilizing reusable goods within a hospital setting. Perhaps the best known is the steam autoclave, which uses high temperature and high pressure steam to render medical goods sterile. Steam as a sterilizing gas is fast and effective, but requires high temperatures. Goods that cannot withstand autoclaving temperatures can be sterilized with sterilizers using biocidal gases such as ethylene oxide or hydrogen peroxide.
Alternatively, some types of medical goods are suited to being sterilized by a liquid bath. A commercial example of a liquid sterilization process utilizes the Steris(trademark) System 1 Processor and Steris(trademark) 20 sterilant (peracetic acid) available from the Steris Corporation of Mentor, Ohio.
Regardless of the method and sterilizing agent selected, an important part of the process of providing sterile goods is the verification that the sterilization has been effective. There are two broad classes of indicators that are used to make this verification. The first class includes biological indicators; these devices include viable spores of particularly hardy stains of bacteria. After the sterilizing cycle, the user places the biological indicator in an environment conducive to bacterial growth. If no growth occurs, it is presumed that the cycle was effective. The second class includes chemical indicators; these devices include a portion coated with a chemical such as an indicator ink which undergoes a visible change when subjected to the pre-determined lethal environment that the sterilizer is designed to create within the sterilizing chamber. If the visible change occurs, an effective cycle is presumed.
Preferably, chemical indicators show a first color very uniformly across their indicating region up until the moment all the pathogens on the goods have been destroyed. At that point the indicating region would change all at once to a dramatically different, but still very uniform color. Practically, this is a very difficult objective to achieve, especially with a liquid sterilant. Optionally, the product may have a failsafe built in to provide early warning of sterilizer failure.
The art remains interested in ways to provide very distinct, uniform transitions particularly for liquid sterilization procedures. A chemical indicator for use with a liquid bath sterilization cycle is subjected to different design constraints than a chemical indicator for a vapor sterilant procedures such as steam or ethylene oxide.
A chemical indicator is commercially available for the Steris liquid peracetic acid sterilization process from the Steris Corporation of Mentor, Ohio. The indicator ink associated with the product, however, is very small and difficult to see and read.
A clearly visible, uniform final color after a successful sterilization cycle is particularly desirable. Medical practitioners prefer not to subjectively judge the degree of color change against a visual standard. This is due in part to inherent human variability and subjectivity, such as degrees of color blindness, and a desire to reduce the costs associated with personnel training.
The Commission Internationale de l""Elcairage (CIE) has defined a uniform and objective color measurement method. The method includes definitions of the following parameters:
a standard illuminant;
a standard observer; and
a uniform color space.
The standard illuminant does not refer solely to the light (which may be called the xe2x80x9csourcexe2x80x9d) but also includes a definition for the surrounding environment and the geometry of the measurement system. Four major types of illuminant are defined and named A, B, C, D and represent varying conditions of simulated incandescence or daylight. For general purposes, the de facto illumination is the standard illuminant xe2x80x98A.xe2x80x99
The CIE also defines a standard observer, intended to represent the color perception of the average human viewer of a sample under the standard illuminant. The CIE defines what are called the xe2x80x9ctristimulusxe2x80x9d values for the standard observer. Also necessary when describing human color interpretations is a definition on which area of the eye the color is impinging. The CIE defines the xe2x80x9c2xc2x0 standard observer,xe2x80x9d representing the central, most color-sensitive portion of the interior eye (called the fovea). It is understood that by combining the tristimuli in varying quantities, all colors can be numerically described as they would strike the fovea of a human eye.
The CIE also defines a uniform color space, represented in a chromaticity diagram called the CIE 1976 L*a*b* (CIELAB). The CIELAB chart describes colors along three axes:
L*: describing black to white on a scale of 0 to 100
a*: describing green to red
b*: describing blue to yellow
This chromaticity diagram allows a directional difference between colors to be noted. A change of 1-2 CIELAB units is generally defined as the industrial tolerance. By definition, this means that a one unit change in any one of the L*, a* or b* measurements is considered visually discernible.
The CIE system provides a measurement system based on a shared color space. The CIELAB formulas, which are based on the best available approximations to a uniform color space, are used for many color-difference measurements.
Over the years, many companies and laboratories in a variety of fields have developed methods that allow them to specify their color tolerances and reproducibly evaluate them. Of these methods, there is one that is more precise and quite extensively employed. The method involves the use of a spectrophotometer. A spectrophotometer uses a tightly calibrated standard illuminant and measures the reflectance/transmittance of a sample across a broad spectrum of xe2x80x9cvisiblexe2x80x9d light (roughly 400-700 nm). Using the mathematical transformations developed by the CIE, the computer generates the L*a*b* values for the standard observer. This is a simple, yet very powerful tool for the evaluation of color samples.
The present invention provides a sterilization indicator that presents a visually distinct and uniform color transition after being subjected to a liquid sterilization procedure. The sterilization indicator has a base layer having an indicator compound associated with at least an indicating region thereof. The indicator also has a raised portion (preferably a lamina) attached to the base layer so as to substantially surround the indicating region. No theory is fully understood or advanced as to the precise reasons why the present invention is helpful with a liquid bath sterilization cycle. Surprisingly, a raised portion, conveniently in the form of an annulus around the indicating region, is helpful for use in a chemical indicator for monitoring a liquid peracetic acid sterilization cycle.
In another aspect, the invention provides an objective test for evaluating the uniformity of color in a chemical sterilization indicator.